IPA07-SG-005
PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-First Annual Convention and Exhibition, May 2007 THE PETROLEUM SYSTEM OF SIBOLGA BASIN BASED ON CORRELATION SEISMIC AND WELL LOG DATA Beiruny Syam* Wahyu Permana* Aulia Pradana* Tiggi Choanji*
ABSTRACT
Sibolga Basin is a fore-arc basin, located in northwest of North-Sumatera Province. The borders of Sibolga Basin are Andaman Sea at north, Nias Island at south, Sumatera Island at east, and Hindia Ocean at west. Directly, structural pattern that was developed in Sibolga Basin is related to Semangko Fault and Mentawai Fault. This fault developed since Mesozoikum and continued to tertiary, then reactivated in Pliocene. The stratigraphy succesion at Sibolga Basin can be divided into five lithology units, i.e. Basal Unit, Clastic Miocene Unit, Carbonate Middle-Miocene Unit, Carbonate LateMiocene Unit, and Pliocene-Plistocene Unit. Based on seismic and well log data: Source rocks of Sibolga Basin are Basal Miocene Clastic unit, Carbonate Middle Miocene, and pre-Neogene sedimentary rocks. Reservoirs of Sibolga Basin are Carbonate Middle Miocene, transgressive unit I, Carbonate Unit II, and transgrassive unit II. Stratigraphic and structural traps are seen in seismic and well log data. Reef carbonate in Carbonate Unit II, Middle Miocene to Pliocene, is a stratigraphic and structural traps shown by anticline shape which is formed in Pliocene. The result from well log and seismic data shows that location of seal rock in clastic unit upward is pro-delta to inner neritic facieses. Migration at Sibolga Basin occurred in Pliocene to Plistocene period. INTRODUCTION
The location of Sibolga Basin is at north-west of North-Sumatera Province. The borders of Sibolga Basin are Andaman Sea at north, Nias Island at south, Sumatera Island at east, and Hindia Ocean at *
Padjajaran University
west (Figure 1). Sibolga Basin is interesting to be studied because the position of this basin is at forearc zone. Geologically, this basin has an unique characteristic lithology. The purpose of this paper is to know about its mechanism and petroleum system of Sibolga Basin, from the source rock until the migration. In here, we use secondary data (data that has been interpreted by another writer). We reinterpreted this data and created a reconstruction using flattening method. The Sibolga Basin is asymetrical to the southwest with upwards 6100 m of Neogene sediment adjacent to the outer-arc ridge. A high-angle fault zone formed the western margin of the basin and created associated drag structure as dip strike-slip fault that cut diagonally through the basin in the vicinity of Nias-Banyak Islands. In spite of these faults, the majority of the Neogene sedimentary rocks in the basin are un-deformed. Directly, structural pattern that was developed in Sibolga Basin is related to Semangko Fault and Mentawai Fault. This fault developed since Mesozoikum and continued to tertiary, then reactivated in Pliocene. The stratigraphy succesion at Sibolga Basin can be divided into five lithology units, i.e. Basal Unit, Clastic Miocene Unit, Carbonate Middle-Miocene Unit, Carbonate LateMiocene Unit, and Pliocene-Plistocene Unit. Rose (1983) divided stratigraphy of Sibolga Basin into two parts, i.e. pre-Neogene rocks and Neogene sedimentary rocks. This assignment is based on lithology characteristic and age of rocks. The stratigraphic succession displayed in Figure 2. METHODOLOGY
Revealing petroleum system in Sibolga Basin, the previous data has been re-interpreted. Input data that issued in this paper are regional geology, seismic data, and well data.
Flattening method is used to interpret the seismic data. The reason is, because this method can determine basin condition in the time when sedimentation happened. RESULT AND DISCUSSION
Based on flattening, it shows the source of material sediment at Pre-Neogen – Middle Miocene come from north-northeast (Figure 4). As a result, terrestrial clastic material dominated in this stage, i.e. carbonaceous shale, limestone, and mudstone. These lithologies can be good source rocks to generate hydrocarbon, with the characteristic of non-marine – shallow marine and defined as kerogen type II and/or III. In Middle Miocene – Early Pliocene, direction of sedimentation changed from north-east to southwest. This stage started with growing up of carbonate in shelf area. This carbonate can be good as source rock and also as reservoir. That source rock representing kerogen type II and III. Reservoir residing in fracture and in pore that increased as a result of dissolution. Furthermore, transgression was happened and made environmental change from shallow marine to deep marine. In this time, clastic material overlain the carbonate. The clastic material consists of shale, mudstone, and sandstone. This material becomes a source rock, especially from shale that consists of organic matter associated with marine environment. That source rock representing kerogen type II and III. Sandstone can become reservoir with trapping mechanism as stratigraphic trap in inter-fingering between sandstone and shale. In middle Late Miocene, the environment changed to be shallow marine. This condition was the result of uplifted and sea level rise. At this condition, carbonate build-up developed in several places. At this time, deposition of clastic started and the source came from south-western part of basin. Clastic material dominated by shale and filled the space between carbonates and also overlain it. This carbonate can became a good reservoir and shale is a good sealing for prevent hydrocarbon that have been snared can not be migrating to another place. In Pliocene phase until recently, the deposition direction has been changed from north-northeast to
west-southwest. The material deposition was deltaic, which could be seen from lithology characterization and flattening. Sediment material which was deposited in Pliocene period until Pleistocene was a continental material, so this material formed a sealing rock. Hydrocarbon maturity occurred in late Pliocene and then it migrated at that time. CONCLUSION
Based on seismic and well log data: Source rocks of Sibolga Basin are Basal Miocene Clastic unit, Carbonate Middle Miocene, and pre-Neogene sedimentary rocks. Reservoirs of Sibolga Basin are Carbonate Middle Miocene, transgressive unit I, Carbonate Unit II, and transgrassive unit II. Stratigraphic and structural traps are seen in seismic and well log data. Reef carbonate in Carbonate Unit II, Middle Miocene to Pliocene, is a stratigraphic and structural traps shown by anticline shape which is formed in Pliocene. The result from well log and seismic data shows that location of seal rock in clastic unit upward is pro-delta to inner neritic facieses. Migration at Sibolga Basin occurred in Pliocene to Plistocene period. REFERENCES
Beaudry, Desiree and Moore, G.F., 1985, Seismic Stratigraphy and Cenozoic. Evolution of West Sumatera Fore Arc Basin. AAPG Buletin vol 69, p 742-759. Rose, Robet., 1983, Miocene Carbonate Rocks of Sibolga Basin Northwest Sumatera, Proceedings Indonesia Petroleum Association, p 107. Darman, Herman amd Sidi Hasan., 2000, An Outline of The Geology of Indonesia. Indonesian ASSOC of Geologist (IAGI), p 15 – 16. Reijers, T.J.A & Hsu, K.J., 1986, Manual of Carbonate Sedimentology : A Lexicographical Approach,Academic Press. London. Sudarmono, Suherman T., and Eza Benny., 1997, Paleogene Basin Development in Sundaland and It’s Role to The Petroleum Systems in Western Indonesia (ed. J.V.C. Howes and R.A. Noble). Proceeding IPA Petroleum Systems of South East Asia and Australia.
Figure 1 - Location of Sibolga Basin.
Figure 2 - Stratigraphic Column of Sibolga Basin (adapted from Rose (1983)).
Figure 3 - Work Flow of Methodology.
Figure 4 - Basement Condition in Paleogene, Depotition Started from Early Miocene until Middle Miocene. Line seismic direction north-south.
Figure 5 - Basement Condition in Middle Miocene, the deposition started from middle Miocene until late Miocene. Seismic line relative directional north to south.
Figure 6 - Basement condition in age Upper Miocene until Pliocene, the deposition start from Upper Miocene until Pliocene. Seismic line relative directional SouthWest.
Figure 7 - The Petroleum Chart of Sibolga Basin.
